Some important, but less well-known descriptors are the two-point surface correlation functions which arise in rigorous bounds on transport properties of porous media [1]}, [2]}. The interface indicator function is defined as [1]}
\(\mathcal {M}(\mathbf {x})=|\nabla \mathcal {I}^{(1)}(\mathbf {x})|=|\nabla \mathcal {I}^{(2)}(\mathbf {x})|.\) 
